Well fracturing is an often used technique to increase the efficiency and productivity of oil and gas wells. Overly simplified, the process involves the introduction of a fracturing fluid into the well and the use of fluid pressure to fracture and crack the well strata. The cracks allow the oil and gas to flow more freely from the strata and thereby increase production rates in an efficient manner.
There are many detailed techniques involved in well fracturing, but one of the most important is the use of a solid “proppant” to keep the strata cracks open as oil, gas, water and other fluids found in well flow through those cracks. The proppant is carried into the well with the fracturing fluid which itself may contain a variety of viscosity enhancers, gelation agents, surfactants, etc. These additives also enhance the ability of the fracturing fluid to carry proppant to the desired strata depth and location. The fracturing fluid for a particular well may or may not use the same formulation for each depth in the strata.
Proppants can be made of virtually any generally solid particle that has a sufficiently high crush strength to prop open cracks in a rock strata at great depth and temperatures of about 125° C. and higher. Sand and ceramic proppants have proved to be especially suitable for commercial use.
A proppant that is flushed from the well is said to have a high “flow back.” Flow back is undesirable. In addition to closure of the cracks, the flushed proppants are abrasive and can damage or clog valves and pipelines in downstream processing facilities.
Synthetic resin coatings can be used to impart a degree of adhesion to the proppant so that flow back is substantially reduced or eliminated. Such resins can include phenol resin, epoxy resin, polyurethane-phenol resin, furane resin, etc. See published US Patent Application Nos. 2002/0048676, 2003/0131998, 2003/0224165, 2005/0019574, 2007/0161515 and 2008/0230223 as well as U.S. Pat. Nos. 4,920,192; 5,048,608; 5,199,491; 6,406,789; 6,632,527; 7,624,802; and published international application WO 2010/049467, the disclosures of which are herein incorporated by reference.
With some coatings, the synthetic coating is not completely cured when the proppant is introduced into the well. The coated, partially-cured proppants are pourable, but the coating resin is still slightly thermoplastic. The final cure is intended to occur in situ in the strata fracture at the elevated pressures and temperatures found “down hole.”
Unfortunately, partial curing of coatings on sand-sized proppants is extremely difficult to control in a reproducible manner. The stresses on package stacking, the temperatures experienced in warehouse storage during storage and temperature stress upon introduction of the proppants into the strata can all raise the temperature sufficiently to cause uncontrolled post-curing at undesirable times. As one might imagine, such instability and handling difficulties have effectively limited the use of post-cure coatings in proppants for oil and gas wells.
Proppants based on polyurethane chemistries have a number of potential advantages over phenol resin systems. Most notably, the reaction rates used to make polyurethane coatings are generally faster than phenol resins, cure at lower temperatures and do not have gaseous emissions that require specialized recovery equipment. The coating step with polyurethanes can be carried out at temperatures of about 10° C. to about 50° C. Polyurethane coatings can also be performed without the use of solvents, whereas many of the known methods, as a rule, require organic solvents for the resinous coating. The components in polyurethane systems are also generally easier to use and pose lower environmental issues. These factors could reduce the cost to make coated proppants and could also permit the coating process to be moved to the site of the well head.
Polyurethanes have not, however, achieved widespread adoption due to relatively high flow back ratios at the down hole conditions and low coating levels needed to permit the proppant to enter the very small fractures in a stratum. The coated proppant simply flows back up out of the well and does not become lodged therein in sufficient quantities to maintain conductivity.